Surface ionization source

ABSTRACT

An ionization source comprises a housing defining a space, a wire extendingo the space, a temperature control connected to the wire and an alkali metal glass bead attached to the wire to be heated by the temperature control. An ion extraction plate with an orifice therein covers the space and is insulated from the housing by an insulating ring. The housing includes a port for drawing a vacuum from the space and for leaking molecules into the space which are to form quasi ions on the glass bead. A focusing plate may also be provided over the extraction plate with an aperture lying on an ion axis which also extends through the orifice for the focusing of the ion beam.

STATEMENT OF GOVERNMENT INTEREST

The invention described herein may be manufactured, used and licensed byor for the Government for Governmental purposes without the payment tous of any royalties thereon.

FIELD AND BACKGROUND OF THE INVENTION

The present invention relates in general to the field of ion generationand in particular to a new and useful surface ionization source which iscapable of converting neutral molecules into ions containing themolecules.

Many ways are already known for producing gas phase ions for analysis bymass spectrometry. The most commonly used of these methods are electronimpact, in which a beam of energetic (typically 70 electron volts)electrons is passed through a low pressure (less than 10⁻⁴ torr) ofvaporized sample. This was the earliest and is still the most widelyused method of forming ions in commerically produced mass spectrometers.Typically, the electron impact ionization results in the formation of alarge number of fragment ions and in many cases does not produce asignificant abundance of intact molecular ions. The molecular ions areuseful because they provide information on the molecular weight of ofthe sample molecules.

The more recent innovation of chemical ionization forms ions by means ofa gas phase chemical reaction between ions produced from a reagent gas,present in large excess, and the neutral sample molecules. These ionsare typically the sample molecule plus one proton, and the molecularweight of the original sample can be easily deduced by subtracting themass of a proton, namely, 1.0007825, from the measured mass of the MH+ion.

Many other methods of ionization exist but are less commonly used. Amongthese are ionization by vacuum ultraviolet photons, termedphotoionization; ionization by very high electric fields, termed fieldionization; and ionization by bombardment of the sample upon a solidsurface with high energy beams of either ionic or neutral particles.

Mass spectrometers are useful in detecting contaminants in the air. Onesuch application is in the field of chemical warfare where it is desiredto determine whether a chemical is present on a battlefield. Theapplication of mass spectroscopy however is complicated in theenvironment of a battlefield which would be expected to contain othersubstances that would not normally be found in clean air.

A man-portable field alarm system for chemical agents based on theprinciple of tandem mass spectrometry or mass spectrometry/massspectrometry (MS/MS) would be very useful if such a device could beconstructed. Such man-portable field alarm systems would be required todetect trace levels of chemical agents in the presence of largeconcentrations of complex interfering substances, such as diesel fuelsmoke. Based on the results of several space exploration programs, asingle, small, compact, man-portable, battery-operated mass spectrometersystem could easily be produced that is capable of detecting tracelevels of chemical agents in clean air. However, to detect chemicalagents in the presence in high concentrations of interfering mixtures, amore sophisticated MS/MS system is required.

In an MS/MS detector, the first mass spectrometer separates all ionscorresponding to the molecular weight of the chemical agent from thelarge variety of ions produced from the total mixture of substances inthe air. A particular mass-selected ion beam is then fragmented, and thefragment ions separated according to their mass by the second massspectrometer. Because chemical agents have fragmentation patternsdifferent from those of common interfering substances, the presence offragment ions characteristic of chemical agents can be used to detecttheir presence, even in complex mixtures.

Limitations to the portability of mass spectrometers are size, weight,and power consumption. An examination of commercially available, smallmass spectrometers, which are principally residual gas analyzers, showsthat by far the largest single limiting factor is powerconsumption--specifically the vacuum system and its associated pumps.Conventional vacuum pumps use two separate stages of pumping to maintaina vacuum against the ambient atmosphere. The first stage is either adiffusion pump or a turbomolecular pump. This in turn is backed in thesecond stage by a mechanical vacuum pump, usually called a forepump.Even if the newer miniature turbomolecular pumps are used, which arelightweight and low in power consumption, the smallest availablemechanical pump will push the power requirement for the vacuum pumpsalone to well over 500 watts. Even if the weight of these items could betolerated, 500 watts is well outside the capabilities of anyman-portable battery pack.

An alternative vacuum pump, called a triode pump, ionizes gas by acontinuous DC discharge. Pumping occurs by the burial of ions in atitanium cathode as well as by chemical reactions of neutral gasmolecules with sputtered titanium metal. In operation, the powerconsumed by these pumps is well under 1 watt (this is as long as the gasload remains small; otherwise power would be 10 to 40 W); the majordeterminant of their power consumption is thus the efficiency of theirrequired high-voltage power supply. Furthermore, once started, thesepumps do not require backing by a mechanical forepump.

In use, the entire vacuum system, including the triode pump, isevacuated, baked and then sealed off. As long as the gas load on thevacuum system stays within the capacity rating of the triode pump, thesedevices can maintain a high vacuum without any mechanical pumprequirements. Small triode pumps suitable for a portable massspectrometer system have pumping speeds of 1-5 1/s and throughputs atlow power consumption of up to about 10⁻⁴ Torr 1/s. The limited pumpingcapacity available means that ionization methods such as chemicalionization (CI) (not necessarily true in the case of the Finnigan Iontrap however), and atmospheric pressure ionization (API), both of whichintroduce large gas loads, would not be feasible for a man-portable massspectrometer system. Commercially available systems would therefore notbe practical for the portable field alarm detector.

SUMMARY OF THE INVENTION

The present invention provides a new means for producing gas phase ionsfor subsequent analysis by mass spectrometry or other methods. This newsource of ions is extremely simple to construction, very inexpensive toproduce, and is inherenty rugged. This invention is based upon the factthat many materials have ionic species naturally present in substantialconcentrations on their surfaces. When neutral molecules are caused tocollide with such surfaces, they may form complexes on the surface withthe previously existing ionic species. At appropriate temperatures, thecomplex between these ionic species and the neutral molecule willevaporate from the surface as a gas phase ionic complex. For example,molecules of acetone, which have a molecular formula of C₃ H₆ O,colliding with a surface containing a high concentration of potassiumcations, K+, are converted into ions with the formula C₃ H₆ OK+, andwould appear in the mass spectrum at a mass equal to the sum of themolecular weight of acetone (58) and the atomic weight of potassium(39), that is 58+⃡=97.

The present invention is particularly suited as an ion source for amass-portable field alarm system. The alarm system incorporating thepresent invention can be referred to as a surface ionization massspectrometer/mass spectrometer or SIMS/S. Further work has been shownthat organic alkali complexes produced by the surface ionization sourceproduce only an alkali ion when they undergo either collision induceddissociation or surface induced dissociation. As a result, there is nocharacteristic spectra produced for the second mass analyser to scan.##STR1##

The simple surface ionization source of the invention has beenconstructed and tested. The source comprising the following parts:

A surface ionization element, which is, for example, a piece of 0.005inch diameter irridium wire bent into the shape of a hair pin. A smallbead of potassium or sodium glass is supported at the bend in the hairpin. The temperature of the glass bead is controlled by varying anelectrical current passed through the irridium wire.

A source body or housing which serves to confine the sample molecules inthe vicinity of the surface ionization element.

An ion drawout plate, which provides a means of conveying the ionsformed on the surface ionization element to the mass spectrometer.

An insulating ceramic ring which allows a difference in electricalpotential to be maintained between the source housing and the iondrawout or extraction plate. A focus electrode can also be provided toproduce appropriate electrical fields for focusing the resulting ionflux into a nearly parallel beam.

The inventive source has been tested with ketones, aromatichydrocarbons, alkylphosphonates and olefins. In all cases the only ionformed is the complex between the neutral molecule and the potassiumion.

It has been well known for many years that certain substances arespontaneously converted into ions upon hitting a hot filament in avacuum. For example, if a beam of neutral potassium atoms strikes a hotfilament of tungsten, iridium, or other metal with high work function, asignificant portion of the incident atoms are converted to positiveions. This phenomenon has been termed surface ionization.

In contrast to earlier surface ionization devices, however, the presentinventions converts incident neutral molecules into ions through acomplexation with ions previously existing on the surface of theionization element. The ionic species formed is not simply the incidentparticle with one electron removed. It is therefore a qualitativelydifferent process from the earlier surface ionization method, and has amuch broader range of applicability, since it is expected to be capableof producing molecular complex type ions from almost all chemicalcompounds.

The examples produced to date of the surface ionization element consistsof various glasses which are heated by means of passing an electricalcurrent through their supporting filament structure. These are onlyspecific examples of possible surface ionization elements. Any materialwhich can have ionic species present on its surface can be used inprinciple as a surface ionization element in this invention. Thematerial is not limited to glasses but could be materials such aspolycrystalline ceramics, metal alloys, or even appropriate hightemperature stable polymers. Any appropriate means may be used tocontrol the temperature of the surface ionization element. Thetemperature at which the maximum yield of ions is obtained from aparticular molecule can be expected to vary with the composition of thesurface ionization element.

It is not intended to limit this invention to the production of ions formass spectrometry. In this specific application the ions are separatedaccording to their mass in a mass spectrometer which is used as a meansof characterizing the sample molecules. Other methods for detecting theions formed by this invention need not employ any form of mass analysis.The ion current might be detected with a simple electrometer circuit, orspecific ions might be detected by other means, such as laser inducedfluorescence.

Accordingly an object of the present invention is to provide a surfaceionization source which comprises a surface ionization element made atleast partly of a material which has ionic species present on itssurface, temperature control means connected to the surface ionizationelement for controlling the temperature thereof to produce high yieldsof ions of particles of a target molecule which strikes the surface, asource housing defining a space at least partly surrounding the elementfor confining ions therein, the housing having a vacuum port for drawinga vacuum from the port, an ion extraction plate engaged with the housingfor closing the space, the extraction plate having an orifice throughwhich a stream of ions can pass and insulating means between the housingand the extraction plate for electrical insulation therebetween.

A further object of the invention is to provide a surface ionizationsource which is simple in design, rugged in construction and economicalto manufacture.

The various features of novelty which characterize the invention arepointed out with particularity in the claims annexed to and forming apart of this disclosure. For a better understanding of the invention,its operating advantages and specific objects attained by its uses,reference is made to the accompanying drawings and descriptive matter inwhich a preferred embodiment of the invention is illustrated.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is a sectional view showing the surface ionization source of thepresent invention;

FIG. 2 is a greatly enlarged partial sectional view of an ionizationelement used in accordance with the invention, illustrating theoperating principle of the invention;

FIG. 3 is a block diagram showing the use of the inventive ionizationsource in a dual mass spectrometer arrangement;

FIG. 4 is a graph showing concentration of a known chemical agent (DIMP)plotted against ion counts generated by the ion source of the presentinvention and also another ion source which was used in determining theadvantages of the present invention;

FIG. 5 is a graph showing the DIMP quasi-molecular ion spectrum obtainedusing a laboratory size MS/MS system; and

FIG. 6 is a graph showing the sensitivity of a surface ionization sourceas shown in FIG. 2 where a sodium glass bead is used for half mustard.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to the drawings in particular, the invention embodied in FIG.1 comprises an active surface ionization source generally designated 50.Such a source can be useful in a mass spectrometer arrangement as shownin FIG. 3.

FIG. 3 shows the general concept of a proposed chemical alarm systembased on the inventive ionization mass source and a mass spectrometry.The ionization source 50 converts the molecules of chemicals present inthe air into ions. These ions are separated according to their mass bythe first mass spectrometer 60, labeled MS(1). A particularmass-selected ion is transmitted to the next stage 62 of the device,where it is broken into fragments. The second mass spectrometer 70,labeled MS(2), separates the fragments according to their mass andproduces a fragment ion spectrum 80 or pattern characteristic of theoriginal molecule. Thus, MS(1) separates the various chemicals enteringthe device after they are converted into molecular ions; ionfragmentation and MS(2) characterize and identify individual chemicals.For highly specific detection of a particular compound, ions of thecorrect molecular weight are fragmented, and MS(2) is set to monitor achemically characteristic fragment of that compound. This works fine forEI and FI but not with this source.

As shown in FIG. 2, gas phase molecules of the target agent simulants GAimpinge upon the surface of potassium glass bead 18, that is rich inalkali cations, shown as K+. On the surface, complexes form betweenthese cations and the neutral molecules. These complexes aresubsequently desorbed from the high-temperature (800°-1000° C.) surface.

As shown in FIG. 1, the active surface ionization element 10 has a1-mm-diameter bead 18 of alkali metal glass located at the bend in ahairpin 16 of 0.005-inch-diameter iridium wire. When this bead is heatedby passing an electrical current through the wire, large quantities ofalkali metal ions evolve. These ions are removed from the interior ofthe ion source by the drawout plate 24 and focused by a focuselectrodeor plate 28 for subsequent analysis by MS(1). When vapors of variouschemical compounds are introduced into the ion source, additional ionsare seen at the masses corresponding to complexes between the alkalication and the neutral molecule. Housing 20 and plates 24, 28 are biasedby electrical biasing control 40 to form an ion beam on a beam axispassing through openings 26 and 30, and to focus that beam.

FIG. 4 shows the sensitivity of a potassium glass surface ionizationsource for DIMP. FIG. 4 also shows four points (triangles) correspondingto the detection of DIMP by a volcano-style field ionization source (notshown). At concentrations above 1.0 ppm, the field ionization sourcerapidly loses sensitivity. In contrast, the surface ionization source islinear in response over three orders of magnitude of DIMP concentration.The lower limit of detection in this case is determined by the range ofthe device used for generating known concentrations of DIMP vapor inair. FIG. 5 shows the quality of the DIMP quasimolecular ion spectrumobtained using the large laboratory version of the MS/MS systemoperating with the quadrupole MS(1) set to transmit all ions. Theexcellent signal-to-noise and good mass resolution show that the DIMPspectrum could easily be detected at much lower DIMP concentration inair.

The exact mode of formation of the complexes between the alkali metalions and neutral molecules as shown in FIG. 2 is not well established.To obtain additional information about this mechanism, a glass beadionizer (not shown) was constructed in which a miniature thermocouplewas spot welded to the irridium wire hairpin 16 at exactly the point atwhich the alkali metal glass bead 18 was attached. This arrangementallowed the inventors to monitor the ion current as a function of thetemperature. Plotting the natural logarithm of the ion signal versus oneover the absolute temperature of the bead allowed the inventors toobtain, in the usual way, the activation enthalpy for the formation ofthe ionic species observed. For a sodium glass, which produces Na+ ions,the activation enthalpy was between 81 and 85 kcal/mole. This was truefor the production of the sodium ion alone or the sodium ion complexwith either benzofuran or bis-2-chloroethyl ether. Because theactivation energies are so similar, it was impossible to distinguishbetween the process that forms the alkali metal complex ion on thesurface and one in which the alkali metal complex ion forms in the gasphase.

Some compounds however clearly do react on the surface of the glassbead. The major product ion observed when adding benzylchloride to theion source was at m/z 91, corresponding to the benzyl cation. Apparentlya chloride is extracted from the benzylchloride to produce this benzylcation. The activation enthalpy we observed for the overall process forforming m/z 91 was about 48 kcal/mole. Similarly when we addedchloroethylethyl sulfide (half mustard) to the sodium glass bead ionsource, the only product ion we observed was at m/z 89, againcorresponding to abstraction of a chloride from the molecule. Wemeasured the activation enthalpy for this process at approximately 50kal/mole, substantially different from that for the formation of thesodium cation and its complexes. The formation of these chlorideextraction ions must therefore be occurring on the surface of the hotglass bead.

FIG. 6 shows the sensitivity of detection of a sodium glass bead surfaceionization source for half mustard when the chloride abstraction production was monitored at m/z 89. Air saturated with half mustard vapor wasinjected into an exponential dilution flask to produce an initialconcentration of half mustard of 2.5 ppm. A flow of 1 L/min of clean airthrough the bulb this flask produced an exponential dilution of the halfmustard vapor in the air exiting the bulb and blowing over the air inletsystem of the laboratory MS/MS system. The quadrupole MS(1) (FIG. 3) wasoperated in the rf-only mode to allow transmission of all ions, andMS(2) was set to monitor the product ion at m/z 89. The response of theion source and mass spectrometer system was linear in half mustardconcentration over almost four orders of magnitude. At half mustardconcentrations below 1 ppb, the inventors observed substantial tailingof the decay curve, caused by adsorption of the half mustard on thesurfaces of the vapor generation apparatus.

The alkali metal glass surface ionization source has been sufficientlytested at this point to demonstrate that it is a viable alternative tofield ionization for use in a portable mass spectrometer system.

It is noted that thermonic emitters consisting of alkali-doped glasseshave been used recently for chemical ionization of organic samples. SeeB. Ackerman et al., Proc. 3lst Annual Conference on Mass Spectrometryand Allied Topics, Boston, May 1983; pp. 600-601; and D. Bombick, J. D.Pinkston, J. Allison, Anal. Chem. 1984, 56, 396-402. Source pressures inthe range of 20 to 700 millitorr are typical, requiring an instrumentwith differential pumping. It is possible to obtain reasonable ionsignals at lower sample pressures with the bead functioning as a surfaceionization source.

An organic-alkali surface ionization source like the one shown in FIG. 1has been constructed and evaluated using an alkali glass bead of thetype R₂ O:Al₂ O₃ :SiO₂ in a 1:1:2 molar ratio mounted on the irridiumheater wire with the bead centered on the ion axis. Mass analysis wasdone on the MS/MS previously described. Vapors of the samples in air orN₂ were introduced into the source through a membrane separator kept at100° C. or directly by means of a variable leak at port 22. Sourcepressures varied from 8×10⁻⁷ torr to 8×10⁻³ torr. When the glass beadwas heated to 800° to 1000° C. by temperature control 14, organic-alkaliions of the formula [M+X]⁺ were seen in addition to large currents ofthe alkali itself, where X=Li, Na, K, Rb, or Cs. At higher temperatures,the intensity of the organic-alkali complex dropped sharply while thealkali ion intensity increased further, indicating a surface effectrather than chemical ionization. This drop in ion current at highertemperatures has also been observed for surface ionization of simpleorganics on hot metal wires. However, if an irridium wire without a beadis used, no ions are seen unless it is heated sufficiently to emitelectrons, resulting in an EI spectrum.

Polar or polarizable compounds seem to complex most easily with theorder: ketones, ethers, alcohols>alkenes>simple aromatics>alkanes. Theorder for complexing with simple aromatics and alkanes was observed tobe Li>Na>K>Rb. For more polar compounds, Na and K glasses work well;pyrex glass also works, giving predominantly Na ion adducts.

Some compounds, such as 2-chloro ethyl-ethyl sulfide and benzyl chlorideundergo abstraction to form ions with no addition of alkali. Benzylalcohol produces both a [M+Na]+ at m/z 131 and a loss of OH to producem/z 91. CID with argon on m/z 131 produces only m/z 23 (Na), while CIDon m/z 91 results in a fragmentation spectrum similar to toluene. Most[M+Na]⁺ or [M+K]⁺ yield only Na⁺ or K⁺ with CID, hence this technique isnot useful for MS/Ms. For one series of experiments, note above, thesodium glass bead with a thermocouple imbedded in the glass was used todirectly read the bead temperature. With this technique it is possibleto determine the heat of formation for alkali ions and alkali-orqaniccomplexes by using the Arrhenius equation: ##EQU1## where T is thetemperature in °K, R equals 1.987 cal mol⁻¹, and k is the ion countrate. Table 1 lists the heats of formation for several ions obtained inthis manner.

While a specific embodiment of the invention has been shown anddescribed in detail to illustrate the application of the principles ofthe invention, it will be understood that the invention may be embodiedotherwise without departing from such principles.

What is claimed is:
 1. A surface ionization source comprising:a sourcehousing defining an ion space; a support wire extending into said space;temperature control means connected to said wire for heating said wire;an alkali metal glass bead connected to said wire to be heated to formions of molecules in said space; an ion extraction plate engaged to saidhousing and insulated from said housing, said extraction plate closingsaid space and having a beam orifice therein for the passagetherethrough of an ion beam from said space; and biasing means connectedto said housing and said extraction plate for forming a beam on a beamaxis extending through said orifice.
 2. A surface ionization sourceaccording to claim 1 wherein said glass bead includes one of sodium orpotassium.
 3. A surface ionization source according to claim 1 includinga focusing plate engaged over said extraction plate and having anaperture therein lying on said beam axis, said focusing plate beingconnected to said biasing means for focusing the beam of ions on saidbeam axis.
 4. A surface ionization source according to claim 3 whereinsaid glass bead includes one of sodium or potassium.
 5. A surfaceionization source according to claim 4 including an insulating ceramicring engaged to said housing and extending around said surface, saidring being engaged against said extraction plate for closing saidsurface and for insulating said extraction plate from said housing.